Nitride semiconductor device having a zinc-based substrate

ABSTRACT

A nitride semiconductor device includes a semiconductor substrate; a first nitride semiconductor layer provided on the semiconductor substrate; a mask layer having opening portions, provided on the first nitride semiconductor layer; a second nitride semiconductor layer selectively grown on the mask layer laterally from the opening portions; and a semiconductor lamination portion formed by laminating nitride semiconductor layers so as to form a semiconductor element on the second nitride semiconductor layer. The substrate may be made of a zinc-based compound, the first nitride semiconductor layer may be provided on, and in contact with, the substrate, and at least a substrate side of the first nitride semiconductor layer may be made of Al y Ga 1-y N (0.05≦y≦0.2). Additionally, the semiconductor element may be a light emitting layer in which case the mask layer may include a metal film provided on the first nitride semiconductor layer and an insulating film provided on the metal film.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device using nitridesemiconductor crystal layers, such as a semiconductor light emittingdevice like a light emitting diode (LED), a laser diode (LD) or thelike, or a transistor device like a HEMT or the like, using nitridesemiconductor. More particularly, the present invention relates to anitride semiconductor light emitting device with excellent crystallinityand high external quantum efficiency even when light emitted in a lightemitting layer is absorbed by using an electric conductive semiconductorsubstrate, and relates to a nitride semiconductor device in whichnitride semiconductor layers with excellent crystallinity are grown byusing an electric conductive ZnO based compound for a substrate, and bypreventing a surface of a substrate from being roughened by etching thesubstrate with a raw material of group V element for growing the nitridesemiconductor layers, while using a MOCVD (metal organic chemical vapordeposition) method which makes mass production easy.

BACKGROUND OF THE INVENTION

In recent years, nitride semiconductor light emitting devices such as ablue light emitting diode (LED), a laser diode (LD) or the like, usingnitride semiconductor, have been in practical use. As shown, forexample, in FIG. 9( a), the LED emitting blue light using nitridesemiconductor is formed by laminating, a low temperature buffer layer 52made of GaN or the like, and a semiconductor lamination portion 56 whichincludes an n-type layer 53 made of GaN or the like, an active layer(light emitting layer) 54 made of, for example, InGaN based (which meansthat a ratio of In to Ga can be varied variously and the same applieshereinafter) compound semiconductor which has a smaller band gap energythan that of the n-type layer 53 and decides a wavelength of emittedlight, and a p-type layer 55 made of GaN or the like on a sapphiresubstrate 51 by the MOCVD method. And a p-side electrode 58 is providedon a surface thereof interposing a light transmitting conductive layer57 and an n-side electrode 59 is provided on a surface of the n-typelayer 53 exposed by etching a part of the semiconductor laminationportion 56. In this case, a semiconductor layer having still larger bandgap energy such as an AlGaN based (which means that a ratio of Al to Gacan be varied variously and the same applies hereinafter) compound orthe like may be used on the active layer side of the n-type layer 53 andthe p-type layer 55 in order to increase an effect of carrierconfinement (cf. for example PATENT DOCUMENT 1).

However, since an electrode can not be formed directly on the substratebecause the sapphire substrate 51 is an insulating substrate, it isnecessary, as described above, to form a mesa structure by a processsuch as etching a part of the semiconductor lamination portion 56 or thelike, then a vertical type device in which a pair of electrodes isformed on both sides of a chip can not be obtained. In addition, sincelattice constants of sapphire and nitride semiconductor materials arevery different from each other, dislocation density increases by latticemismatching, a semiconductor device with high quality can be hardlyobtained. Furthermore, thermal conductivity of the sapphire substrate 51is lower comparing with a substrate made of GaAs, GaP, Si or the likewhich is conventionally used for a conductive substrate of a red orinfrared semiconductor light emitting device, or a SiC substrate or thelike which is used for a blue semiconductor light emitting device. Then,there is suggested an idea of a structure in which a nitridesemiconductor light emitting device is formed by using suchsemiconductor substrates in place of the sapphire substrate.

More concretely, as shown in FIG. 9( b), the nitride semiconductor lightemitting device is formed by laminating a buffer layer 62 made of AlGaNbased compound or the like, and a semiconductor lamination portion 66formed by laminating an n-type layer 63, an active layer (light emittinglayer) 64, and a p-type layer 65, on a Si substrate 61 by a MOCVDmethod, and by providing a p-side electrode 68 on a surface thereofinterposing a light transmitting conductive layer 67, and an n-sideelectrode 69 directly on a back surface of the Si substrate 61.

-   PATENT DOCUMENT 1: Japanese Patent Application Laid-Open No.    H10-173222 (cf. FIG. 1)

DISCLOSURE OF THE INVENTION Problem to be Solved by the PresentInvention

As described above, by forming a nitride semiconductor light emittingdevice directly on the substrate made of GaAs, GaP, Si, SiC or the likein place of sapphire, a vertical type device in which a pair ofelectrodes is formed on both sides of a chip can be produced, and lightemitting efficiency under a high temperature and high output can beimproved because the substrate has a thermal conductivity higher thanthat of the sapphire substrate.

However, since a wavelength of light emitted by a nitride semiconductorlight emitting device is in a range from yellow to ultraviolet light,when light emitted in the light emitting layer of the semiconductorlamination portion travels to a direction of the substrate and reachesthe substrate, if the above-described substrate is used, the light isabsorbed at the substrate, and there arises a problem such that a lightemitting device with high efficiency of taking out light can not beformed. In addition, if a substrate made of the above-describedmaterials is used, because difference of a lattice constant between thesubstrate and those of nitride semiconductor layers laminated thereon islarge, a dislocation density in the nitride semiconductor layers on thesubstrate is high, then, there also arises a problem such that anefficiency of light emitting can not be improved.

On the other hand, there is an idea such that absorption of light at thesubstrate is prevented by using a silicide substrate which is formed byvapor deposition of W or the like, for example on a Si substrate.However, even in such case, since a reflection coefficient at a silicideportion is not so high, light transmits to the substrate side, and lightis absorbed at the substrate after all, then efficiency of taking outlight can not be enhanced. In addition, even if the substrate made ofsilicide is formed by using W, since the substrate reacts with ammoniagas which is a raw material for a GaN based compound laminated thereon,at the time of MOCVD growth, and an impurity layer is formed at aninterface between the silicide and an AlGaN based compound layer,crystallinity of a light emitting layer laminated thereon deteriorates,the efficiency of taking out light is lowered.

Then, a structure using a ZnO substrate which can be formed so as tohave a lattice constant similar to that of a nitride semiconductormaterial and electric conductivity may be suggested in place of using asapphire substrate. However, when it is intended to use a ZnO substrateand grow a nitride semiconductor lamination portion thereon by using aMOCVD apparatus, the nitride semiconductor lamination portion is usuallygrown at a high temperature of, concretely, 1,000° C. or more by usingan organic metal for a raw material of group III element and ammonia gasfor a raw material of group V element. However, the ammonia gas has afunction of etching a surface of the ZnO substrate under a hightemperature condition, therefore the surface of the ZnO substrate isroughened by the ammonia gas just before growing the nitridesemiconductor lamination portion on the ZnO substrate, and thereoccasionally occurs deterioration of crystallinity of the nitridesemiconductor lamination portion grown thereon, or film separationbetween the nitride semiconductor lamination portion and the substrate.On the other hand, in order to inhibit the above described problem,there is an idea such that the nitride semiconductor lamination portionis formed at an extremely low temperature of, concretely, 600° C. orless for preventing the surface from being roughened, however, in a lowtemperature growth, since crystal axes are not orientated in the samedirection and crystallinity deteriorates, crystal defects are generatedin the nitride semiconductor lamination portion and light emitted in alight emitting layer is absorbed, and also since an invasion rate ofimpurities into films increases, electric resistances of the films grownbecome high. In such manner, if the nitride semiconductor laminationportion is grown on a ZnO substrate by the MOCVD method, nitridesemiconductor layers with excellent quality can not be obtained in bothcases of a high temperature and a low temperature.

The present invention is directed to solve the above-described problemsand an object of the present invention is to provide a nitridesemiconductor device with low leakage current and high characteristicsin which, while a zinc oxide based compound such as Mg_(x)Zn_(1-x)O(0≦x≦0.5) is used for a substrate, crystallinity of nitridesemiconductor grown thereon by a MOCVD method which is superior in massproduction is improved and film separation and cracks are prevented.

Another object of the present invention is to provide a nitridesemiconductor light emitting device having high luminous efficiency, byusing a semiconductor substrate enable to produce a vertical type devicein which a pair of electrodes is formed on both sides of a chip, enableto prevent light absorption by the substrate while maintaining highthermal conductivity, and also enable to reduce dislocation density of anitride semiconductor layer grown on the substrate.

Still another object of the present invention is to provide asemiconductor light emitting device such as a LED, a LD or the likehaving a structure capable of improving light emitting characteristicssuch as external quantum efficiency by laminating nitride semiconductorlayers while using a zinc oxide based compound such as Mg_(x)Zn_(1-x)O(0≦x≦0.5) for a substrate.

Means for Solving the Problem

The present inventor studied earnestly and repeatedly for growingnitride semiconductor layers on a ZnO based compound substrate by aMOCVD method, and, as a result, found together with other inventors anddisclosed in PATENT APPLICATION NO. 2005-305596 that by carrying outcontrolling a temperature during growth, controlling a ratio of flowrates of raw gasses for growth, specifying a principal plane of the ZnObased compound substrate on which the nitride semiconductor layers aregrown, growing an AlGaN based compound layer containing Al on a surfaceof the substrate, or the like, the nitride semiconductor layers can begrown without roughening the surface of the ZnO based compound substrateso much, and after covering the surface of the ZnO based compoundsubstrate with the nitride semiconductor layers, even if a temperatureof the substrate is raised, the substrate can be prevented from beingroughened by ammonia gas. However, if a first nitride semiconductorlayer is thin, when a temperature is raised to a high temperature ofapproximately 1,000° C. for growing a nitride semiconductor layer withhigh quality, ammonia gas invades a substrate side, then the substrateis roughened and crystallinity of the first nitride semiconductor layerdeteriorates. In addition, if the first nitride semiconductor layer isthick, a period for growing the first nitride semiconductor layerbecomes long depending on variations of process parameters, thereby thesurface is occasionally roughened, and also separation from thesubstrate occasionally occurs which is caused by difference of thermalexpansion between the nitride semiconductor layer and the ZnO basedcompound substrate.

Then, as a result of further earnest and repeated studies, the presentinventor found that conditions of forming a first nitride semiconductorlayer firstly provided on a ZnO based compound substrate are set so thatthe ZnO based compound is not invaded by ammonia gas with the aboveconditions, the layer is formed with a thin layer of a thickness inwhich the ZnO based compound substrate is not influenced even if theconditions are varied to some extent, and a mask layer is formed thereonwhich is made with a dielectric film having opening portions, therebyeven if a GaN based compound layer is formed at a high temperaturethereafter, epitaxial growth is carried out laterally on the mask layerfrom the opening portions, and, as a result, a nitride semiconductorlayer with excellent crystallinity can be grown. Namely, since thedielectric film made of SiO₂ or the like for the mask layer can beformed at a low temperature, the ZnO based compound substrate is notinfluenced at all, and whole region except the opening portions can becovered. And, at the opening portions, the ZnO based compound substrateis covered with the first nitride semiconductor layer, and an area ofeach of the opening portions is very small, then the ZnO based compoundsubstrate is hardly roughened even when exposed to an atmosphere ofammonia gas at a high temperature of 1,000° C. or more.

Here, the zinc oxide (ZnO) based compound semiconductor means an oxideincluding Zn, and means concretely besides ZnO, an oxide of one or moreelements of group IIA and Zn, an oxide of one or more elements of groupIIB and Zn, or an oxide of elements of group IIA and group II B and Zn.And, the nitride semiconductor means a compound of Ga of group IIIelement and N of group V element or a compound (nitride) in which a partor all of Ga of group III element substituted by other element of groupIII element like Al, In or the like and/or a part of N of group Velement substituted by other element of group V element like P, As orthe like, and is referred to as GaN based compound. In addition, a zincoxide based compound, for example Mg_(x)Zn_(1-x)O, has a hexagonalcrystal structure as its schematic perspective view is shown in FIG. 5,a C plane is a (0001) plane of a Zn polarity plane and a (000-1) planeof an O polarity plane, as shown in FIG. 5, and any of them is a planeorthogonal to an A plane {11-20} and an M plane {10-10}. In addition,(000-1), (11-20), (10-10), {11-20} and {10-10} mean strictly

(000 1), (11 20), (10 10), {11 20} and {10 10},

however, an abbreviated notation is used as described above inconvenience. In addition, for example, a {11-20} plane means a generalterm meaning including planes equivalent to a (11-20) plane bysymmetricity of crystals.

A nitride semiconductor device according to the present inventionincludes: a substrate made of a zinc oxide based compound; a firstnitride semiconductor layer provided on the substrate; a mask layerhaving opening portions, provided on the first nitride semiconductorlayer; a second nitride semiconductor layer selectively grown on themask layer laterally from the opening portions; and a semiconductorlamination portion formed by laminating nitride semiconductor layers soas to form a semiconductor element on the second nitride semiconductorlayer.

Concretely, by forming the first nitride semiconductor layer at asubstrate temperature of 600 to 800° C. by a MOCVD method and with athickness of 500 to 8,000 Angstroms, the first nitride semiconductorlayer with good quality is obtained, and since the first nitridesemiconductor layer is protected with the mask layer on a surfacethereof, the substrate is protected to growth at a high temperature andthe first nitride semiconductor layer excellent for seeds can be exposedat the opening portions.

It is preferable that at least a substrate side of the first nitridesemiconductor layer is made of Al_(y)Ga_(1-y)N (0.05≦y≦0.2), from theviewpoint of preventing more a surface of the ZnO substrate from beingroughened, as described above. Further, it is preferable that aprincipal plane of the substrate is a (0001) plane and Zn polarityplane, also from the viewpoint of preventing more a surface of the ZnOsubstrate from being roughened, as described above.

Concretely, an n-type layer, an active layer and a p-type layer arelaminated on the second nitride semiconductor layer so as to form alight emitting layer, thereby a semiconductor light emitting device isformed.

Another embodiment of a nitride semiconductor light emitting deviceaccording to the present invention includes: a semiconductor substrate;a first nitride semiconductor layer provided on the semiconductorsubstrate; a mask layer having opening portions, provided on the firstnitride semiconductor layer; a second nitride semiconductor layerselectively grown on the mask layer laterally from the opening portions;and a semiconductor lamination portion formed by laminating nitridesemiconductor layers so as to form a light emitting layer on the secondnitride semiconductor layer, wherein the mask layer includes a metalfilm provided on the first nitride semiconductor layer and an insulatingfilm provided on the metal film.

In addition, it is preferable that the metal film is formed with atleast double layer structure of a first metal film provided on the firstnitride semiconductor layer and a second metal film provided on thefirst metal film, and the first metal film is made of a metal which hasa melting temperature higher than a growth temperature of thesemiconductor lamination portion and the second metal film is made of ametal which reflects light emitted in the light emitting layer.

Concretely, the metal film may be used which is formed with the firstmetal film made of at least one of W, Ti and Pd, and the second metalfilm made of at least one of Al, Ag and Au.

EFFECT OF THE INVENTION

By the nitride semiconductor device according to the present invention,since the first nitride semiconductor layer and the mask layer havingopening portions are laminated on the substrate made of a ZnO basedcompound such as Mg_(x)Zn_(1-x)O or the like and the second nitridesemiconductor layer is selectively grown on the mask layer laterallyfrom the opening portions using the first nitride semiconductor layer asseeds, even if the first nitride semiconductor layer is thin, the ZnObased compound substrate is covered with the mask layer made of SiO₂ orthe like, then even if the layers are exposed to an ammonia gasatmosphere of a high temperature for growing at high temperature, thesubstrate can not be eroded. Furthermore, since, by providing the masklayer, the first nitride semiconductor layer grown directly on the ZnObased compound substrate can be formed thin, a period of being exposedto the ammonia gas atmosphere can be shortened, then, as describedabove, by carrying out controlling a temperature during growth,controlling a ratio of flow rates of raw gasses for growth, specifying aprincipal plane of the ZnO based compound substrate on which the nitridesemiconductor layers are grown, growing an AlGaN based compound layercontaining Al on a surface of the substrate, or the like, the substrateis prevented from being roughened perfectly and the first nitridesemiconductor layer with excellent crystallinity can be grown.

As a result, even if the substrate is exposed to an ammonia atmosphereat a high temperature of 1,000° C. or more in order to grow the secondnitride semiconductor layer with excellent crystallinity, since thesubstrate is never eroded, and, additionally the second nitridesemiconductor layer can be grown laterally on the mask layer using thefirst nitride semiconductor layer with excellent crystallinity as seeds,the second nitride semiconductor layer is also formed in a semiconductorlayer with excellent crystallinity, and, furthermore, the nitridesemiconductor layers further laminated thereon can be also formed withvery excellent crystallinity. As a result, since, in the nitridesemiconductor layers laminated on the second nitride semiconductorlayer, light is not absorbed and films with excellent crystallinity inwhich crystal axes are orientated in the same direction can be formed,thereby useless impurities in the films decrease and the films of a lowelectric resistance with high carrier density and electron mobility canbe formed. Then, even when a LED, a LD or the like is formed, asemiconductor light emitting device with excellent characteristicshaving a low operation voltage, high internal quantum efficiency, and alow threshold current can be obtained as a vertical type device in whicha pair of electrodes is taken out from upper and down sides of a chip,and when a transistor or the like is formed, a transistor (HEMT) with ahigh speed having a small leakage current and a high withstand voltagecan be obtained. In addition, by increasing a mixed crystal ratio of Mgof the substrate, band gap energy is increased, and light with a shortwavelength such as ultraviolet light or the like is not absorbed.

In addition, by the nitride semiconductor light emitting deviceaccording to the present invention using other semiconductor substrate,even in case of using a semiconductor substrate which absorbs blue orultraviolet light such as a substrate made of GaAs, GaP, SiC or thelike, since a metal film which is easy to reflect light is formed underthe insulating film, the light generated in a light emitting layer isreflected by the metal film, the light is prevented from reaching thesubstrate, and absorption of the light at the substrate is inhibited,thereby a semiconductor light emitting device capable of enhancingefficiency of taking out light can be obtained. And, for increasing ofdislocation density caused by lattice mismatching, since the secondnitride semiconductor layer is formed by lateral selective growth byusing a mask layer having opening portions and using the first nitridesemiconductor layer exposed at opening portions as seeds, penetratingdislocations are inhibited by the mask layer, and a layer with lessdislocations and high crystallinity can be obtained. As a result, alsoin a light emitting layer laminated thereon, dislocations decreases, aproblem of lattice mismatching is solved, and the light emitting layerwith small dislocation density and high quality can be formed. Inaddition, since the substrate is made of semiconductor, a conductivesubstrate can be formed by doping, then, in case of forming LED, it isnot necessary to form a mesa structure, and a vertical type device inwhich a pair of electrodes is formed at upper and down sides of a chipcan be obtained, and since the thermal conductivity of the semiconductoris higher than that of the sapphire, the thermal saturation is inhibitedup to a high temperature and high output, thereby a semiconductor devicewith high light emitting efficiency can be obtained.

In addition, by forming the metal film with a two layer structure of afirst metal film made of a metal which reflects light generated in alight emitting layer, and a second metal film made of a metal which hasa melting point higher than a growth temperature of a semiconductorlamination portion, the metal of the first metal film can be preventedfrom diffusing into semiconductor layers, therefore, characteristics ofthe device is not deteriorated, and absorption of light at the substratecan be inhibited sufficiently, and, as a result, a semiconductor lightemitting device with higher light emitting efficiency can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an explanatory cross-sectional view of a LED which is anembodiment of the nitride semiconductor device according to the presentinvention; and FIG. 1B is a partial sectional view illustrating aprotection film formed on the substrate of FIG. 1A.

FIG. 2 is an enlarged explanatory cross-sectional view of a vicinity ofthe mask layer according to the present invention.

FIG. 3 is an explanatory cross-sectional view of an example which isanother structure of the nitride semiconductor device according to thepresent invention.

FIG. 4 is an explanatory cross-sectional view of a constitution of thetransistor formed by the present invention.

FIG. 5 is a figure for explaining a ZnO crystal structure.

FIG. 6 is an explanatory cross-sectional view of a LED which is anotherembodiment of the nitride semiconductor device according to the presentinvention.

FIG. 7 is an enlarged explanatory cross-sectional view of a vicinity ofa mask layer by an example shown in FIG. 6.

FIG. 8 is an explanatory cross-sectional view of the nitridesemiconductor device by the example shown in FIG. 6 before elements aredivided.

FIG. 9 is figures of examples of constitutions of LEDs usingconventional nitride semiconductor.

EXPLANATION OF LETTERS AND NUMERALS

1: ZnO substrate

2: first nitride semiconductor layer

4: mask layer

5: second nitride semiconductor layer

9: semiconductor lamination portion

31: semiconductor substrate

32: first nitride semiconductor layer

33: metal film

34: insulating film

35: mask layer

36: second nitride semiconductor layer

40: semiconductor lamination portion

THE BEST EMBODIMENT OF THE PRESENT INVENTION

An explanation will be given below of a nitride semiconductor deviceaccording to the present invention in which a ZnO based compoundsubstrate is used as a substrate in reference to the drawings. As anexplanatory cross-sectional view of a nitride semiconductor lightemitting device (LED chip) of an embodiment is shown in FIG. 1A, thenitride semiconductor device according to the present invention isformed such that a substrate 1 is made of a zinc oxide based compoundsuch as Mg_(x)Zn_(1-x)O (0≦x≦0.5), a first nitride semiconductor layer 2is provided on the substrate 1, a mask layer 4 having opening portions 4a and a second nitride semiconductor layer 5 selectively grown laterallyfrom the opening portions 4 a are formed on the first nitridesemiconductor layer 2, and nitride semiconductor layers 6 to 8 arelaminated on the second nitride semiconductor layer 5 so as to form asemiconductor element (so as to form a light emitting layer of a LED inthe example shown in FIG. 1A). In addition, in the example shown in FIG.1A, the first nitride semiconductor layer 2 is formed by a first layer 2a made of AlGaN based compound provided at a substrate 1 side and asecond layer 2 b made of GaN provided at an upper side, and, although aplurality of layers are used in this case, single layer may be used. Inaddition, substrates in all figures including FIG. 1A, are drawn thincomparing to other semiconductor layers, however, actually, a thicknessof the substrate 1 is much larger than that of each semiconductor layer.

Namely, the present invention is characterized in using a substrate madeof a zinc oxide based compound such as Mg_(x)Zn_(1-x)O or the like as asubstrate 1, and providing the first nitride semiconductor layer 2directly on a surface of the substrate, the mask layer 4 having openingportions on the first nitride semiconductor layer 2, and the secondnitride semiconductor layer 5 epitaxially grown laterally on the masklayer 4, in order to laminate nitride semiconductor layers by a MOCVDmethod. As described above, when the nitride semiconductor layers aregrown by the MOCVD method, it is preferable to grow at a temperature ofapproximately 1,000° C. or more because high quality of a GaN film canbe obtained at a high growth temperature, however, if a ZnO basedcompound substrate is used as the substrate 1, the ZnO substrate isetched by ammonia gas, a surface of the substrate 1 where epitaxialgrowth is carried out is roughened, and the nitride semiconductor layerswith high quality of a film can not be grown. On the other hand, whenthe growing is carried out at a low temperature of 600° C. or less inorder to prevent the above-described problem, quality of a film of GaNdeteriorates, and there arises a problem such as deterioration ofcrystallinity of the nitride semiconductor layers.

However, the present inventor discovered, as a result of earnest andrepeated studies as described above, that, by growing a thin firstnitride semiconductor layer 2 while controlling a temperature or thelike so as to prevent a ZnO based compound substrate from beingroughened, and forming the mask layer 4 having opening portions with adielectric film made of SiO₂ or the like, the substrate is protected bythe mask layer 4, then, even under an ammonia atmosphere of a hightemperature, the ZnO based compound substrate can be prevented frombeing roughened, and by growing the second nitride semiconductor layerepitaxially grown laterally on the mask layer 4 at a high temperaturethereafter, a nitride semiconductor device with excellent crystallinitycan be formed. Namely, by adopting such manner, it was found that thereare solved the problem such as roughness of the substrate caused byvariation of process parameters generated when increasing a thickness ofthe first nitride semiconductor layer 2, or occurrence of cracksdepending on a difference of the thermal expansion between the nitridesemiconductor layer and the substrate 1, because although the mask layer4 has the opening portions, most area of a surface of the substrate 1 iscovered by the mask layer 4, and then, even if a thickness of the firstnitride semiconductor layer 2 underlying is thinned, the surface of thesubstrate 1 is prevented from being roughened.

As the substrate 1, a zinc oxide based compound such as Mg_(x)Zn_(1-x)Oor the like, for example, an n-type ZnO substrate 1, is used. By usingsuch oxide, the substrate can be easily removed by wet etching, oneelectrode can be taken out from a back surface of the substrate sincesemiconductor has conductivity, and lattice matching can be easilyachieved (a crystal layer for seeds is formed with high quality) becausea lattice constant thereof is similar to that of a nitride semiconductorlayer above all, thereby, a film can be formed with higher quality thanthat in a conventional case using a sapphire substrate. In case offorming a light emitting device emitting light with a short wavelength,the substrate 1 may be made of Mg_(x)Zn_(1-x)O (0≦x≦0.5) or the like inwhich Mg is mixed so as not to absorb the light, in place of being madeof ZnO. However, it is not preferable that a concentration of Mg is over50 at % since MgO is a crystal of a NaCl type which does not match witha ZnO based compound of a hexagonal system in lattice. By the way, theMg_(x)Zn_(1-x)O substrate is formed by cutting out wafers from an ingotformed by a hydrothermal synthesis method or the like.

In addition, it is preferable to use a (0001) plane and Zn polarityplane shown in FIG. 5 as a principal plane of the substrate 1, becauseresistance to ammonia gas is high comparing with a principal plane of anO polarity plane, and a surface of the ZnO substrate 1 is lessroughened, however, other planes may be used. Namely, in case of using aC plane as a principal plane of a ZnO substrate, an O polarity plane anda Zn polarity plane exist on the C plane, however, in case of using a Znpolarity plane as the principal plane, Zn appears on a surface, therebyresistance to etching by ammonia gas is higher comparing to the case inwhich O exists on the surface, and roughness of the surface caused byammonia gas is lowered comparing to the O polarity plane.

As described above, since a surface of the substrate 1 is etched byammonia gas when being exposed to an ammonia atmosphere under a hightemperature, the surface is roughened, crystallinity of the substrateitself deteriorates, and, at the same time, crystallinity of nitridesemiconductor layers grown thereon deteriorates remarkably. Then, thenitride semiconductor layers are preferably formed by protecting a backsurface, side and an end portion of the surface of, for example, a ZnOsubstrate 1 a, by coating with a protection film 15 made of SiO₂, Si₃N₄,Pt or the like which does not vaporize at a high temperature as shown inFIG. 1B, and setting the ZnO substrate 1 a (wafer) on a work carrier ofa MOCVD apparatus, made of carbon, molybdenum or the like.

The first nitride semiconductor layer 2 is a layer, which is made ofnitride semiconductor having a lattice constant similar to that of thesubstrate, for inhibiting etching by ammonia gas and, at the same time,for seeds for epitaxial growth in a lateral direction from openingportions of the mask layer 4 described later, and is provided in contactwith the substrate. In order to lower activity and absolute quantity ofammonia gas and to prevent the substrate from being etched, it ispreferable that the first nitride semiconductor layer 2 is formed, incase of growing by a MOCVD method, at a low temperature of 600 to 800°C. which is lower than a usual growth temperature of a GaN crystallayer, and with setting a molar ratio of a raw material of group Velement to that of group III element to 500 or more and 2,000 or less.In addition, in case of forming one electrode on a back surface of thesubstrate 1, a conductivity type of the first nitride semiconductorlayer 2 is required to be the same conductivity type as the substrate 1,however, in case of not forming one electrode on the back surface of thesubstrate 1, the first nitride semiconductor layer 2 may be formedundoped or doped with Si (n dopant) or the like.

It is preferable that a thickness of the first semiconductor layer 2 is500 Angstroms or more in order to prevent ammonia gas from transmittingthe first nitride semiconductor layer, surely. In addition, as describedlater, since, by providing a mask layer 4 having opening portions 4 a,the ammonia gas can not pass through the mask layer 4 in a region exceptthe opening portions 4 a, the thickness of the first nitridesemiconductor layer 2 is not required to be so thick, and can beapproximately 500 to 8,000, preferably 1,000 to 4,000 Angstroms,concretely. In such manner, since the first nitride semiconductor layer2 can be formed thin, there can be inhibited a leakage current byoccurrence of film separation or cracks caused by stress generatedbetween the first nitride semiconductor layer 2 and the substrate 1.

In addition, an AlGaN based compound which has comparatively small Alconcentration is preferably used for the first nitride semiconductorlayer. It is because, ammonia gas can be prevented from reaching thesubstrate and etching the substrate by existence of Al, and even byusing a usual growth method (high temperature growth) for forming asemiconductor lamination portion thereafter, nitride semiconductorlayers with excellent cryatalinity can be grown. More concretely, if GaNor an InGaN based compound is used for the first nitride semiconductorlayer 2, ammonia gas occasionally transmits a layer made of the GaN orthe InGaN based compound because In or the like is apt to vaporize, andthere arises a case such that a surface of the ZnO substrate underlyingis roughened. However, when an AlGaN based compound is used for thefirst nitride semiconductor layer 2, since the first nitridesemiconductor layer 2 contains Al, ammonia gas can be prevented fromreaching a surface of the substrate by existence of Al, and,furthermore, since film adhesion strength of a layer made of the AlGaNbased compound is stronger comparing to that of a layer made of GaN andthe InGaN based compound, film separation hardly occurs.

Therefore, once the first nitride semiconductor layer 2 is formed ofAlGaN with an Al concentration and a film thickness of a certain valueor more, film separation hardly occurs, and at the time of laminating asemiconductor lamination portion under a high temperature conditionthereafter, since ammonia gas does not reach a surface of the ZnOsubstrate, nitride semiconductor layers with excellent crystallinity canbe grown even by using a usual growth method. In addition, sincedifference of a coefficient of thermal expansion with the substrate issmaller than that in a case of GaN or InGaN, an occurrence probabilityof leakage current caused by occurrence of cracks can be lowered. And,it is preferable to set Al concentration of the AlGaN based compound to20% or less and 5% or more. However, as shown in FIG. 1, by forming thefirst nitride semiconductor layer 2 with a plurality of layers or agradient layer, a layer made of GaN or an InGaN based compound may beprovided at an opposite side of the substrate 1 without any problems,and since the mask layer 4 is provided on the most area of the surface,a single layer made of GaN or an InGaN based compound may be used.

In the example shown in FIG. 1, the first nitride semiconductor layer 2is formed with a plurality of layers in which a first layer 2 a made ofan AlGaN based compound is provided at a side of the substrate 1, asecond layer 2 b made of GaN is formed at a surface side, and acomposition of the layer of the surface side is arranged so as to matchto that of a layer laminated on the mask layer 4. In this case, thesecond layer 2 b of the surface side of the first nitride semiconductorlayer 2 becomes a seed layer for lateral growth.

The mask layer 4, as an enlarged explanatory figure of the vicinitythereof is shown in FIG. 2, is provided on the first nitridesemiconductor layer 2, has opening portions 4 a having a width W, and isformed so as to contact directly with the first nitride semiconductorlayer 2. The mask layer 4 is formed by depositing a dielectric materialor the like such as, for example, SiO₂, Si₃N₄ or the like, on which asemiconductor layer can not be epitaxially grown directly, by asputtering or CVD method, with a thickness of approximately 200 to 800nm. The thickness of 200 nm or more is required for preventing ammoniagas from invading a substrate, and that of 800 nm or less for preventingcrystals from deterioration caused by occurrence of level differences.

The mask layer 4 is provided on whole surface of the first nitridesemiconductor layer 2 of a wafer state, thereafter the opening portions4 a are formed by patterning (extending with a groove shape in adirection perpendicular to a paper surface of FIG. 1 or 2). A reason whythe opening portions are provided is that, the first nitridesemiconductor layer 2 exposed at the opening portions 4 a acts as seeds,the second nitride semiconductor layer 5 is selectively grown laterallyon the mask layer 4, thereby a dislocation density of the second nitridesemiconductor layer 5 is lowered. In addition, in order to form a LED ofa type in which light is emitted from an upper surface, between the masklayer 4 and the first nitride semiconductor layer 2 at regions exceptthe opening portions, a metal film made of Al, Ag, Au or the like may beprovided as a reflection layer.

When a semiconductor light emitting device shown in FIG. 1 ismanufactured, a width M of the mask layer is set to 10 to 15 μm in orderto achieve laterally selective growth. If an interval W of the masklayer is too large, longitudinal growth in which a dislocation densityis large occurs, therefore it is preferably set to 5 μm or less. Inaddition, if the interval W of the mask layer is too small, it takeslong time to grow crystals from the first nitride semiconductor layer 2,therefore it is preferably 2 μm or more.

In addition, in order to make dividing into each device easy, if, asshown in FIG. 1, only a mask layer of a portion (both end portions of anelement (chip)) of a region for dividing into each element is spreadcomparing to other portions, laterally selective growth does not occursurely on the mask layer 4 of the portion of the region for dividinginto each element, and the wafer is divided spontaneously by the portionof the region of the mask layer 4 for dividing into each element. Then,the width of the mask layer near the region for dividing into eachelement is formed wider than that of the mask layer near a centerportion of the element, and preferably set concretely to 40 to 80 μm. Bysuch constitution, since each element can be independent before formingthe semiconductor lamination portion 9, even if a crack by a stressoccurs in one element, spread of the crack to other elements can beinhibited. In addition, even if a stress depending on difference of acoefficient of thermal expansion acts between the substrate 1 and thesemiconductor lamination portion, the stress is absorbed in the regionfor dividing into each device, and a wafer itself can not be bent in anarcuate shape.

The second nitride semiconductor layer 5 is formed with, for example, ann-type GaN layer and with a thickness of approximately 5 to 10 μm. Thesecond nitride semiconductor layer 5 begins to grow using the first GaNlayer 2 b exposed from the opening portions 4 a of the above-describedmask layer 4 as seeds, and when it reaches a surface of the mask layer4, it grows selectively in a lateral direction. Namely, since a GaNlayer grows faster and with more excellent crystallinity in a lateraldirection than in a longitudinal direction, by growing slightly in alongitudinal direction while growing in a lateral direction,semiconductor layers growing in a lateral direction from both openingportions are joined each other at the vicinity of a center portion ofthe mask layer 4 at last. Then, after a surface of the mask layer 4 iscovered entirely, the semiconductor layer grows in an upper directionand the second n-type GaN layer (semiconductor layer) 5 grows also onthe mask layer 4 entirely. The second n-type GaN layer 5 has excellentcrystallinity at a region except both end portions (portions in contactwith the opening portion 4 a) and a joint portion of the center portionon the mask layer, and a dislocation density is low by one order.

The semiconductor lamination portion 9 on the second n-type GaN layer 5is formed as a semiconductor lamination portion constituting a usuallight emitting diode. Namely, in an example shown in FIG. 1, thesemiconductor lamination portion 9 is formed by providing an n-typelayer 6 made of n-type GaN doped with Si having a thickness ofapproximately 1 to 10 μm, an active layer 7 made with a MQW structure(multiple quantum well structure formed by laminating 3 to 8 pairs ofwell layers made of, for example, In_(0.17)Ga_(0.83)N and having athickness of 1 to 3 nm, and barrier layers made of In_(0.01)Ga_(0.99)Nand having a thickness of 10 to 20 nm) of an undoped InGaN basedcompound and GaN, having a thickness of approximately 0.05 to 0.3 μm intotal, and a p-type layer 8 made of GaN doped with Mg having a thicknessof approximately 0.2 to 1 μm.

In addition, the semiconductor lamination portion 9 is laminated with anecessary constitution depending on a semiconductor device manufactured,and, also in case of a LED, not being limited to the above-describedexample, the n-type layer 6 and the p-type layer 8 may be formed in amulti-layer structure provided with a layer (barrier layer) having alarge band gap energy at a side of the active layer, or a super latticestructure or a gradient layer may be provided between semiconductorlayers having different compositions, or the second nitridesemiconductor layer 5 may share with an n-type layer or a p-type layer.In addition, a structure of the active layer 7 may be a bulk structureor a single quantum well (SQW) structure, not limited to the multiquantum well structure. Further, although the example shows a doublehetero junction structure formed by holding the active layer 7 with then-type layer 6 and the p-type layer 8, a hetero junction structureformed by joining an n-type layer and a p-type layer directly may beused. The point is that the n-type layer 6 and the p-type layer 8 areprovided so as to form a light emitting layer in case of constituting aLED. In addition, although the above-described example is an example ofa LED, a LD can be formed similarly by forming a light emitting regionhaving a stripe shape.

Subsequently, an explanation of a method for manufacturing the lightemitting diode will be given below. A wafer, in which a protection filmis provided on a region except a growth surface of the ZnO substrate 1formed with, for example, an n-type conductivity, and with a principalplane of a (0001) plane and Zn polarity plane, is set within a MOCVDapparatus, and the surface of the substrate is cleaned in an hydrogencarrier gas at a raised temperature of 600 to 800° C., for example 700°C. Subsequently, by supplying ammonia gas (NH₃) of a raw gas of group Velement, and trimethyl gallium (TMG) and trimethyl aluminium (TMA) ofgroup III element, the first layer 2 a of a first nitride semiconductorlayer 2 made of Al_(y)Ga_(1-y)N (0.05≦y≦0.2, for example y=0.2) is grownwith Si doping and with a thickness of 500 Angstroms, for exampleapproximately 2,000 Angstroms, and the second layer 2 b made of GaN isgrown with Si doping and with a thickness of approximately 2,000Angstroms. Here, flow rates of the ammonia gas and the carrier gascarrying the raw material of group III element are adjusted so as to seta molar ratio of the raw materials of group V element and group IIIelement to 2,000 or less, for example approximately 500 (the rawmaterial of group V element of 2×10⁻² mole and the raw material of groupIII element of 4×10⁻⁵ mole). Although the Si doping is necessary forforming an electrode on a back surface of the substrate 1, an undopedsubstrate may be used in case of not forming an electrode on a backsurface of the substrate. It is preferable for preventing the surface ofthe ZnO substrate from being roughened that an atmosphere within achamber is made with an atmosphere of a raw material of group IIIelement at first by supplying TMA and TMG of an organic metal of a rawmaterial of group III element for several seconds just before growingthe first nitride semiconductor layer 2, thereby the protection film isformed on the surface of the ZnO substrate with the raw material ofgroup III element, thereafter, ammonia of a raw material of group Velement is supplied.

And subsequently, the substrate is taken out from the growth apparatus,by using, for example, a sputtering apparatus, a vapor depositionapparatus or the like, a SiO₂ film for the mask layer 4 is formed with athickness of approximately 200 to 800 nm. Thereafter, a resist film isprovided thereon and patterned, and the SiO₂ film is etched by anaqueous solution of HF, thereby the opening portions 4 a are formed witha stripe shape and the mask layer 4 with the stripe shape is formed.

Thereafter, the substrate is set within the MOCVD apparatus or the like,necessary gasses are supplied such as trimethyl indium (TMIn) as a rawmaterial gas for In besides the above-described gas, andcyclopentadienyl magnesium (Cp₂Mg) or dimethyl zinc (DMZn) as a p-typedopant, with hydrogen gas as a carrier gas, thereby the second n-typeGaN layer 5 and each semiconductor layer of the semiconductor laminationportion 9 are grown with each thickness described above. In this case,since the n-type GaN layer 5 which is the second nitride semiconductorlayer 5 is easy to grow laterally when a temperature of the substrate ishigh, and easy to grow longitudinally when a temperature of thesubstrate is low, the layer is grown firstly at a temperature ofapproximately 850 to 1,000° C. and at a temperature of approximately 950to 1,100° C. after the opening portions are filled, the n-type layer 6is grown at a temperature of the substrate of approximately 950 to1,100° C., the active layer 7 is grown at a temperature of the substrateof approximately 700 to 770° C., and each layer after them is grown at atemperature of the substrate of approximately 950 to 1,100° C. again. Inaddition, in order to change compositions of In or Al of an InGaN basedcompound or an AlGaN based compound, flow rates of TMIn of a rawmaterial gas of In and TMA of a raw material gas of Al are adjusted.

Thereafter, a light transmitting conductive layer 10, having a thicknessof approximately 0.01 to 5 μm, which is made of, for example, ZnO or thelike and capable of ohmic contact with the p-type layer 8 is provided ona surface of the semiconductor lamination portion 9. The ZnO is formedin a film so as to have a specific resistance of approximately (3 to5)×10⁻⁴ Ω·cm by doping Ga. The light transmitting conductive layer 10 isnot limited to ZnO, and an ITO film or a thin alloy film of Ni and Auhaving a thickness of 2 to 100 nm can diffuse electric current to wholeof a chip while transmitting light.

Then, after polishing a back surface of the substrate 1 so that athickness of the substrate 1 is approximately 100 μm, an n-sideelectrode 12 is formed by laminating Ti/Al or Cr/Pt/Au or Ni/Au or thelike on the back surface, further a p-side electrode 11 is formed with alamination structure made of Ti/Au by a lift off method on a surface ofthe light transmitting conductive layer 10, and whole of a chip iscovered with a SiON film not shown in the figure by a plasma CVD methodand an opening portion is formed at an electrode portion. Thereafter, alight emitting device chip having a structure shown in FIG. 1 is formedby dividing a wafer into chips.

According to the present invention, since nitride semiconductor layersare laminated on the ZnO based compound substrate, one electrode can beformed on a back surface of the substrate, and a device of a verticaltype can be formed in which a pair of electrodes is formed at upper andlower sides of the chip. However, even in case of using such substrate,the n-side electrode 12 can be formed on the n-type layer 6 exposed byetching a part of the semiconductor lamination portion 9 laminated, bydry etching, as shown in FIG. 3. By using such structure, a deviceemitting sufficient light can be obtained even if the ZnO substrate 1 orthe AlGaN layer 2 a or the GaN layer 2 b has a high electric resistance.Here, a structure of the semiconductor lamination portion or the like issimilar to that of an example shown in FIG. 1, and the same letters andnumerals are attached to the same parts and an explanation is omitted.

FIG. 4 is an explanatory cross-sectional view showing a transistorconstituted by laminating nitride semiconductor layers with excellentcrystallinity by forming a first nitride semiconductor layer 2 made ofan AlGaN based compound on a surface of the above described ZnOsubstrate 1 and a mask layer 4 having opening portions. In a samecondition as a case of the light emitting device, by using a MOCVDapparatus, after growing firstly the first layer 2 a made of undopedAlGaN and the second layer 2 b made of undoped GaN, of the first nitridesemiconductor layer 2, the mask layer 4 is formed, and the openingportions are provided. Subsequently, necessary organic metal gasses aresupplied in the same manner described above, there are formed, in order,the second nitride semiconductor layer 5 made of undoped GaN, an undopedGaN layer 23 approximately 4 μm thick, an electron transit layer 24 madeof undoped AlGaN based compound approximately 10 nm thick, an n-type GaNlayer 25 approximately 5 nm thick, and the electron transit layer 24 isexposed by etching and removing a part of the n-type GaN layer 25 so asto provide a predetermined interval of approximately 1.5 μm to be a gatelength. And a transistor is constituted by forming a source electrode 26and a drain electrode 27 made with, for example, a Ti film and a Au filmon the n-type GaN layer left with the predetermined interval, and a gateelectrode 28 formed by laminating, for example, a Pt film and a Au filmon a surface of the un-doped AlGaN based compound layer 24. The nitridesemiconductor layers with excellent crystallinity can be formed and atransistor (HEMT) with a small leakage current and a high withstandvoltage can be obtained by forming the nitride semiconductor layer 2 andthe mask layer 4 having the opening portions, on a surface of thesubstrate, and by growing the second nitride semiconductor layer 5thereon.

As described above, according to the present invention using the ZnObased compound, since, while using a zinc oxide based compound such asZnO or the like for the substrate, the first nitride semiconductor layerwhich has a similar lattice constant to that of the substrate and aproperty of not transmitting ammonia gas, and the mask layer havingopening portions are provided on a surface of the substrate in order tolaminate nitride semiconductor layers, etching the substrate by ammoniagas does not occur and forming the semiconductor lamination portion canbe carried out at a high temperature, thereby a nitride semiconductordevice with excellent crystallinity can be formed. As a result, therecan be significantly improved characteristics of a device using nitridesemiconductor such as a nitride semiconductor light emitting device suchas a LED, a LD (laser diode) or the like with excellent light emittingcharacteristics, a nitride transistor such as a HEMT or the like with asmall leakage current and a high withstand voltage, or the like.

Subsequently, an explanation of a nitride semiconductor light emittingdevice according to the present invention in which a conductivesemiconductor substrate such as Si or the like is used as a substrate inplace of the ZnO based compound will given below. As an explanatorycross-sectional view of a nitride semiconductor light emitting device(LED chip) of an embodiment is shown in FIG. 6, the nitridesemiconductor light emitting device according to this embodiment isformed by providing a first nitride semiconductor layer 32 on asemiconductor substrate 31, a mask layer 35 having opening portionsthereon, and a second nitride semiconductor layer 36 selectively grownlaterally on the mask layer 35 from the opening portions and, asemiconductor lamination portion 40 formed by laminating nitridesemiconductor layers so as to form a light emitting layer 38. Here, themask layer 35 is composed of a metal film 33 provided at a side of thefirst nitride semiconductor layer 32 and a dielectric film 34 providedon the metal film 33.

Namely, this embodiment is characterized in forming the mask layer 35with the metal film 33 which reflects light traveling to the substrateside, and the insulating film 34 on which the nitride semiconductorlayer can be selectively grown laterally, besides using a semiconductorsubstrate, which is conductive, capable of forming an electrode on aback surface thereof, and high in thermal conductivity. As describedabove, if a sapphire substrate is used, since it is an insulatingsubstrate, an electrode can not be formed on a back surface of thesubstrate and a mesa structure is required to be used, and since thermalconductivity of the sapphire substrate is low, thermal saturation occursat high temperature and high output operation, and light emittingefficiency can not be enhanced. On the other hand, in order to preventthe above-described problem, if a substrate made of GaAs, GaP, SiC, Sior the like which is a conductive substrate is used, since the substrateis made electric conductive by doping, it is not necessary to form amesa structure and a vertical type device in which a pair of electrodesis formed on upper and lower surfaces of a chip can be formed, and sincethermal conductivity of the substrate is high, light emitting efficiencycan be prevented from lowering at high temperature operation. However, adislocation density caused by lattice mismatching between the substrateand a nitride semiconductor layer increases and absorption of light atthe substrate occurs, thereby there arises a problem such that the lightemitting efficiency lowers.

However, according to the present invention, for increase of adislocation density caused by lattice mismatching, since the mask layer35 having opening portions is used, and the second nitride semiconductorlayer 36 is grown by laterally selective growth using the first nitridesemiconductor layer 32 exposed from the opening portions as seeds,dislocations generated at the substrate side is prevented frompenetrating to upper layers by the mask layer 35. Therefore, a densityof penetrating dislocations becomes very small and a nitridesemiconductor layer with excellent crystallinity can be obtained. As aresult, in a semiconductor lamination portion laminated thereon, sincelayers with excellent crystallinity can be formed, even if alattice-mismatched substrate is used, a dislocation density can belowered comparing to the conventional case. In addition, as for theproblem such that light is absorbed at the substrate, since the metalfilm 33 is formed under the insulating film 34, light generated in thelight emitting layer 38 and traveling to the substrate 31 is reflectedby the metal film 33, and prevented from entering the substrate 31,thereby the light absorption at the substrate 31 can be inhibited.

In an example shown in FIG. 6, an n-type Si substrate is used as thesemiconductor substrate 31, however it is not limited to Si, and SiC,GaAs, GaP or the like may be used. In case such that light such as blueor ultraviolet light emitted in nitride semiconductor layers is absorbedby the substrate, the present invention is specially effective. Sincethe semiconductor substrate can be made conductive by doping, oneelectrode can be taken out from a back surface of the substrate. Inaddition, if any of the above-described materials is used, a latticeconstant thereof does not match with that of GaN, and lattice matchingcan not be obtained, however, by laterally selective growth of the GaNlayer through the mask layer 35 as described above, the second nitridesemiconductor layer 36 with a small dislocation density can be grown onthe mask layer 35. In an example described below, an example of ann-type substrate is explained, however a p-type substrate may be used.

The first nitride semiconductor layer 32 is formed with, for example, anAlGaN based compound layer 32 a grown at a low temperature and a GaNlayer 32 b grown at a high temperature which are approximately 3 μmthick and doped in an n-type, by a usual epitaxial growth method such asa MOCVD method or the like, and it is a layer for relaxation of latticemismatching and for seeds at the time of forming the second nitridesemiconductor layer 36 by laterally selective growth. Now, in theexample shown in FIG. 6, the first nitride semiconductor layer 32 has atwo layer structure in which the AlGaN based compound layer 32 a is alattice mismatching relaxation layer and the GaN layer 32 b is a crystallayer for seeds, however, these layers may have a single layer or asuper lattice structure. In addition, a composition may only be anitride semiconductor layer such as an InGaN based compound or the likebesides an AlGaN based compound and GaN, depending difference of latticeconstants of the substrate used and semiconductor layers laminatedthereon, and a thickness of the film is set according to requirements.

The mask layer 35(351), as an enlarged explanatory figure of thevicinity thereof is shown in FIG. 7, is provided on the first nitridesemiconductor layer 32, and composed of the metal film 33 having openingportions 353 with a width W and provided at the first nitridesemiconductor 32 side, and the insulating film 34 provided on the metalfilm 33.

For the metal film 33, a metal having a large reflection coefficient tolight of a blue to ultraviolet region such as, for example, Al, Ag, Auor the like is preferable. In addition, it is preferable, as shown inFIG. 7, to form the metal film 33 with, at least, a two layer structureof a first metal layer 33 a provided at the first nitride semiconductorlayer 32 side and a second metal layer 33 b provided on the first metallayer 33 a, and form the second metal film 33 b of the above-describedmaterial and the first metal film 33 a of a metal having a melting pointhigher than a growth temperature of a semiconductor lamination portion40, because a material of the second metal film 33 b does not make analloy with semiconductor layer and diffusion thereof can be inhibited.For example, W, Ti, Pd or the like is preferably used for the firstmetal film 33 a. It is most suitable to form the first metal film 33 awith a thickness of 10 to 200 nm and the second metal film with athickness of 10 to 200 nm.

The insulating layer 34 is formed with a thickness of approximately 200nm by a sputtering method or the like by depositing an insulatingmaterial such as, for example, SiO₂, Si₃N₄ or the like, on which asemiconductor layer can not be directly grown epitaxially. Theinsulating layer 34 is for not growing the second nitride semiconductorlayer 36 directly on the first nitride semiconductor layer 32, and ifthe film is formed so as to maintain a mask function, a thickness of thefilm is preferably thinner because a level difference does not occur.

The metal film 33 and the insulating layer 34 are provided on a wholesurface of the first nitride semiconductor layer 32 of a wafer stagesequentially, thereafter opening portions 353 with a groove shape(groove extending in a perpendicular direction to the paper surface inFIG. 7) are formed by patterning. The reason why the opening portions353 are provided is, as described later, to grow the second nitridesemiconductor layer 36 by laterally selective growth using the firstnitride semiconductor layer 32 exposed at the opening portion 353 asseeds, and to lower a dislocation density of the second nitridesemiconductor layer 36.

In case of manufacturing a semiconductor light emitting device shown inFIG. 6, a width M1 (cf. FIG. 8) of the mask layer is formedapproximately 10 to 15 μm wide in order to realize laterally selectivegrowth. Since, when an interval W of the mask layer is too wide, alongitudinal growth with a large penetrating dislocation density occursand a laterally selective growth with small dislocation density does notadvance, and also in order to increase an area of a region reflectinglight generated in a light emitting layer, it is preferable that theinterval is 5 μm or less. In addition, since, when the interval W of themask layer is too small, it takes long time to grow crystals from theseeds of the first nitride semiconductor layer 32, the interval ispreferably 2 μm or more.

In addition, as shown in an explanatory cross-sectional figure beforedividing into devices in FIG. 8, in order to make it easy to divide intodevices, and prevent a wafer from bending depending on difference ofcoefficients of thermal expansion between the Si substrate 31 and thenitride semiconductor lamination portion 40 at a wafer stage, if only amask layer 352 corresponding to an element dividing region 45 (both endportions of a chip when divided into elements (chips)) is widened, amask width M2 of the element dividing region is wider than a mask widthM1 of the mask layer 351 within the element, the laterally selectivegrowth does not occur perfectly on the mask layer 352, and elements aredivided spontaneously by the mask layer 352. Then, it is preferable toform the width M2 of the mask layer 352 in the vicinity of the elementdividing region 45 wider than the mask width M1 of the mask layer 351 inthe center portion of the element, and set concretely approximately 20to 80 μm. In such manner, by widening the width M2 of the mask layer 352of the element dividing region 45, even if difference of a coefficientof thermal expansion between the Si substrate 31 and the nitridesemiconductor layers is large, strain caused by the difference of acoefficient of thermal expansion can be absorbed and the substrate canbe prevented from bending or the like. In addition, since each elementcan be independent, a process of dividing a wafer into chips becomeseasy.

The second nitride semiconductor layer 36 is formed, for example, withan n-type GaN layer with a thickness of approximately 5 to 10 μm. Thesemiconductor layer 36 begins to grow using the first GaN layer 32 bexposed from opening portions 353 of the above-described mask layer 35as seeds, and grows selectively in a lateral direction after the growingGaN layer reaches a surface of the mask layer 35. Namely, since the GaNlayer grows faster and better in crystallinity in a lateral directionthan in a longitudinal direction, it grows slightly in the longitudinaldirection while growing in the lateral direction, and the semiconductorlayers grown in the lateral direction from two opening portions joineach other at the vicinity of a center portion of the mask layer 35.Then, after a surface of the mask layer 35 is covered perfectly, the GaNlayer grows upward, and the second n-type GaN layer (semiconductorlayer) 36 is also grown wholly on the mask layer 35. The second n-typeGaN layer 36 has excellent crystallinity and a smaller dislocationdensity by one order at a region except both end portions (portions incontact with the opening portions 353) above the mask layer 35, and acenter portion where the GaN layers join.

The semiconductor lamination portion 40 on the second n-type GaN layer36 is a semiconductor lamination portion constituting a usual lightemitting diode. Namely, the semiconductor lamination portion in anexample shown in FIG. 6 is formed by providing an n-type layer 37 madeof n-type GaN doped with Si having a thickness of approximately 1 to 10μm, an active layer 38 made with a MQW structure (multiple quantum wellstructure formed by laminating 3 to 8 pairs of well layers made of, forexample, In_(0.17)Ga_(0.83)N and having a thickness of 1 to 3 nm, andbarrier layers made of In_(0.01)Ga_(0.99)N and having a thickness of 10to 20 nm) of an undoped InGaN based compound, having a thickness ofapproximately 0.05 to 0.3 μm in total, and a p-type layer 39 made of GaNdoped with Mg having a thickness of approximately 0.2 to 1 μm.

In addition, the semiconductor lamination portion 40 is laminated with anecessary constitution depending on a semiconductor device manufactured,and in case of a LED, not being limited to the above-described example,the n-type layer 37 and the p-type layer 39 may be formed in amulti-layer structure provided with a layer (barrier layer) having alarge band gap energy at an active layer side, or a super latticestructure or a gradient layer may be provided between semiconductorlayers having different compositions, and the second nitridesemiconductor layer 36 may share with an n-type layer or a p-type layer.In addition, a structure of the active layer 38 may be a bulk structureor a single quantum well (SQW) structure, not limited to the multiquantum well structure. Further, although the example shows a doublehetero junction structure formed by holding the active layer 38 with then-type layer 37 and the p-type layer 39, a hetero junction structureformed by joining an n-type layer and a p-type layer directly may beused. The point is that the n-type layer 37 and the p-type layer 39 areprovided so as to form a light emitting layer in case of constituting aLED. In addition, although the above-described example is an example ofa LED, even in case of a LD, if a structure of a semiconductorlamination portion is formed with a lamination structure for the LD, theLD with excellent crystallinity and small leakage current can beobtained.

Subsequently, an explanation of a method for manufacturing the lightemitting diode will be given below. For example, by using a MOCVDapparatus or the like, thermal cleaning is carried out in a H₂atmosphere by raising a temperature of a substrate to approximately1,100° C. Subsequently, a temperature of the substrate is lowered toapproximately 400 to 500° C., and by supplying ammonia gas (NH₃) of araw gas of group V element, trimethyl gallium (TMG) and trimethylaluminium (TMA) of organic metals of a raw material of group IIIelement, and SiH₄ as an n-type dopant, the first Al_(0.05)Ga_(0.95)Nlayer 32 a of an n-type doped with Si is formed with a thickness ofapproximately 0.01 to 0.05 μm, and the GaN layer 32 b is formed with athickness of approximately 1 to 3 μm by raising a temperature of thesubstrate to approximately 900 to 1,100° C.

And subsequently, the substrate is taken out from the growth apparatus,and by using, for example, a sputtering apparatus or a vapor depositionapparatus, a Ti film with a thickness of approximately 10 to 200 nm, aAg film with a thickness of approximately 10 to 200 nm, and a SiO₂ filmwith a thickness of approximately 200 to 500 nm, are formedsequentially. Thereafter, a resist film is provided on the SiO₂ film andpatterned, and by etching the SiO₂ film by an aqueous solution of HF,the Ag film by an aqueous solution of HCl+HNO₃, and the Ti film by anaqueous solution of HF, opening portions are formed in a stripe shapeand the mask layer 33 with the stripe shape is formed.

Thereafter, the substrate is set within a MOCVD apparatus or the like,necessary gasses are supplied such as trimethyl indium (TMIn) as a rawmaterial gas for In besides the above-described gas, andbiscyclopentadienyl magnesium (Cp₂Mg) or dimethyl zinc (DMZn) as ap-type dopant, together with hydrogen gas as a carrier gas, thereby thesecond n-type GaN layer 36 and each semiconductor layer of thesemiconductor lamination portion 40 are grown with each thicknessdescribed above. In this case, since the n-type GaN layer 36 is easy togrow laterally when a temperature of the substrate is high, and easy togrow longitudinally when a temperature of the substrate is low, thelayer is grown firstly at a temperature of approximately 850 to 1,000°C. and at a temperature of approximately 950 to 1,100° C. after theopening portions are filled, the n-type layer 37 is grown at atemperature of the substrate of approximately 950 to 1,100° C., theactive layer 38 is grown at a temperature of the substrate ofapproximately 700 to 770° C., and each layer thereafter is grown at atemperature of the substrate of approximately 950 to 1,100° C. again. Inaddition, in order to change compositions of In or Al of an InGaN basedcompound or an AlGaN based compound, the flow rate of TMIn of a rawmaterial gas of In or TMA of a raw material gas of Al is adjusted.

Thereafter, a light transmitting conductive layer 41, having a thicknessof approximately 0.01 to 5 μm, which is made of, for example, ZnO or thelike and capable of ohmic contact with the p-type layer 39 is providedon a surface of the semiconductor lamination portion 40. The ZnO isformed in a film so as to have a specific resistance of approximately (3to 5)×10⁻⁴ Ω·cm by doping Ga. The light transmitting conductive layer 41is not limited to ZnO, and an ITO film or a thin alloy film of Ni and Auhaving a thickness of 2 to 100 nm can diffuse electric current to wholeof a chip while transmitting light.

Then, after polishing a back surface of the substrate 31 so that athickness of the substrate 31 is approximately 100 μm, an n-sideelectrode 43 is formed by laminating Ti/Al or Cr/Pt/Au or Ni/Au or thelike on the back surface, further a p-side electrode 42 is formed with alamination structure made of Ti/Au by a lift off method on a surface ofthe light transmitting conductive layer 41, and whole of a chip iscovered with a SiON film not shown in the figure by a plasma CVD methodand an opening portion is formed at an electrode portion. Thereafter, alight emitting device chip having a structure shown in FIG. 6 is formedby dividing a wafer into chips.

According to the present invention, since nitride semiconductor layersare laminated on a semiconductor substrate, one electrode can be formedon a back surface of the substrate, and a device of a vertical type canbe formed in which a pair of electrodes is formed at upper and lowersides of a chip. However, even in case of using such substrate, then-side electrode 43 can be formed on the n-type layer 37 exposed byetching a part of the semiconductor lamination portion 40 laminated, bydry etching. It is the effect of applying the present invention that,even by this structure, since a coefficient of thermal conductivity ofthe substrate is higher than that of a usual sapphire substrate, adevice in which deterioration of light emitting efficiency does notoccur up to a high temperature and high output operation can beobtained.

INDUSTRIAL APPLICABILITY

Characteristics of a light emitting device using nitride semiconductor,such as a LED or a laser diode, and a transistor device such as a HEMTcan be improved, and the nitride semiconductor device can be used inevery kinds of electronic apparatus using the nitride semiconductordevice.

1. A nitride semiconductor device, comprising: a substrate made of azinc oxide-based compound; a first nitride semiconductor layer providedon the substrate; a mask layer having opening portions, provided on thefirst nitride semiconductor layer; a second nitride semiconductor layerselectively grown on the mask layer laterally from the opening portions;and a semiconductor lamination portion formed by laminating nitridesemiconductor layers so as to form a semiconductor element on the secondnitride semiconductor layer, wherein a principal plane of the substrateis a (0001) plane and Zn polarity plane.
 2. The nitride semiconductordevice according to claim 1, wherein the first nitride semiconductorlayer is in contact with the substrate so that the first nitridesemiconductor layer has a substrate side, and at least said substrateside is made up of Al_(y)Ga_(1-y)N (0.05≦y≦0.2).
 3. The nitridesemiconductor device according to claim 2, wherein a protection film isformed on a back surface and sides of the substrate.
 4. The nitridesemiconductor device according to claim 2, wherein the first nitridesemiconductor layer is formed with a thickness of 500 to 8,000Angstroms, at a temperature of 600 to 800° C. by a MOCVD method.
 5. Thenitride semiconductor device according to claim 2, wherein the firstnitride semiconductor layer has an opposite side that is opposite to thesubstrate, and wherein the opposite side is formed with a GaN layer oran InGaN-based compound layer.
 6. The nitride semiconductor deviceaccording to claim 2, wherein the mask layer is formed with a thicknessof 200 to 800 nm.
 7. The nitride semiconductor device according to claim2, wherein portions formed by being separated by the opening portions inthe mask layer have widths that are wider at both ends of the nitridesemiconductor device than widths of portions inside of the nitridesemiconductor device.
 8. The nitride semiconductor device according toclaim 2, wherein an n-type layer, an active layer and a p-type layer arelaminated on the second nitride semiconductor layer so as to form alight emitting layer, thereby forming a semiconductor light emittingdevice.
 9. A nitride semiconductor light emitting device comprising: asemiconductor substrate; a first nitride semiconductor layer provided onthe semiconductor substrate; a mask layer having opening portions,provided on the first nitride semiconductor layer; a second nitridesemiconductor layer selectively grown on the mask layer laterally fromthe opening portions; and a semiconductor lamination portion formed bylaminating nitride semiconductor layers so as to form a light emittinglayer on the second nitride semiconductor layer, wherein the mask layercomprises a metal film provided on the first nitride semiconductor layerand an insulating film provided on the metal film.
 10. The nitridesemiconductor light emitting device according to claim 9, wherein themetal film is formed with at least double layer structure of a firstmetal film provided on the first nitride semiconductor layer and asecond metal film provided on the first metal film, and the first metalfilm is made of a metal which has a melting temperature higher than agrowth temperature of the semiconductor lamination portion and thesecond metal film is made of a metal which reflects light emitted in thelight emitting layer.
 11. The nitride semiconductor light emittingdevice according to claim 10, wherein the first metal film is made of atleast one of W, Ti and Pd, and the second metal film is made of at leastone of Al, Ag and Au.
 12. The nitride semiconductor light emittingdevice according to claim 9, wherein the first nitride semiconductorlayer comprises a lattice mismatching relaxation layer provided at thesubstrate side and crystal layer which acts as seeds for the secondnitride semiconductor layer, provided on a surface of an opposite sideto the substrate.
 13. The nitride semiconductor light emitting deviceaccording to claim 9, wherein widths of portions formed by beingseparated by the opening portions in the mask layer are wider at bothends of the device than widths of portions inside of the device.